A turbocharger may be provided in an engine to improve engine torque or power output density. The turbocharger may include an exhaust driven turbine coupled to a compressor via a drive shaft. The compressor may be fluidly coupled to an air intake manifold in the engine. Exhaust flow from one or more engine cylinders may be directed to a wheel in the turbine causing the turbine to rotate about a fixed axis. The rotational motion of the turbine drives the compressor which compresses air into the air intake manifold to improve boost pressure based on engine operating conditions. A variable nozzle turbine may be used in the turbocharger to control engine boost pressure by varying exhaust flow conditions at a turbine nozzle. In this case, the geometry of the variable nozzle turbine may be varied by adjusting a degree of opening of blades of the turbine nozzle to accommodate a wide range of exhaust flow conditions depending on engine speed and load. For example, operating the turbine at a high efficiency with a low turbine expansion ratio may improve engine fuel economy. In this way, the variable nozzle turbine may improve turbine transient response and engine fuel economy.
The variable nozzle turbine may be susceptible to high cycle fatigue, especially under engine exhaust braking conditions. When operating in an engine exhaust braking mode, a high expansion ratio inside the turbine may generate a strong shock wave between the turbine nozzle and wheel. As a result, a strong excitation may propagate to the turbine wheel due to the shock wave, which may lead to high cycle fatigue failure in the turbine.
To improve turbine efficiency, each nozzle blade of the variable nozzle turbine may be optimized for subsonic or transonic flow conditions. Further, shock conditions may be minimized when each nozzle blade is designed to support transonic flow conditions. However, designing nozzle blades to adapt to both the subsonic and transonic flow conditions to achieve high efficiency and weak shock wave, may be a challenge.
A nozzle vane design for a variable vane assembly is disclosed by Groves in U.S. Pat. No. 9,188,019. Therein, the variable vane assembly includes an annular nozzle ring having a plurality of vanes connected to an actuator ring, and an insert having a tubular portion and a nozzle portion. The tubular portion may be accommodated in a bore in a turbine housing while the nozzle portion extends out radially from one end of the tubular portion and may be axially spaced from the nozzle ring.
However, the inventors herein have recognized potential issues with such a system. For example, the design of the vanes of the vane assembly may lead to an increase in exhaust flow near end walls of the turbine, and a reduction in flow area in a throat formed between each pair of vanes. In this case, total pressure losses in the vane assembly may increase due to an increase in end wall or turbine wheel losses. An increase in pressure losses in the system may adversely affect turbine efficiency and performance.
In one example, the issues described above may be addressed by a nozzle blade for a turbine nozzle of a variable geometry turbine, comprising: a cambered outer surface that curves from an inlet end to an outlet end of the nozzle blade, relative to a chord of the nozzle blade, the chord having a chord length defined from the inlet end to the outlet end, the nozzle blade having an aspect ratio in a range of 1.54 to 2.56, a thickness that is greatest in a range of 47 to 61% of the chord length. In this way, each nozzle blade on the turbine nozzle may be designed to direct exhaust flow into the turbine while reducing end wall and turbine losses.
As one example, nozzle blades having a specified combination of aspect ratio, blade thickness and camber line angle change ratio may be provided on a turbine nozzle of a variable nozzle turbine, thereby allowing the turbine to accommodate a range of exhaust flow conditions depending on engine operating conditions, such as engine speed and load.
The approach described here may confer several advantages. For example, each nozzle blade of the turbine nozzle may be adjustable between an open and a closed position, where a degree of blade opening may be adjustable to accommodate a wide range of exhaust flow conditions based on engine operating conditions. In addition, the nozzle blades of the turbine nozzle may be adapted to have a wide range of aspect ratios, blade thickness and camber line angle change ratios. By providing a turbine nozzle having nozzle blades which each have a combination of an aspect ratio in a specified range, a blade thickness in a specified range, and a camber line angle change ratio in a specified range, turbine efficiency may be improved while reducing high cycle fatigue of the turbine components.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.